1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
Conventionally, there has been used widely a spark plug configured such that a metallic terminal member is inserted into one end portion of a through-hole formed axially in an insulator; a center electrode is inserted into the other end portion of the through-hole; and the metallic terminal member and the center electrode are securely and rigidly held within the through-hole in a sealed condition by use of a conductive seal material. Within the through-hole formed in the insulator, the metallic terminal member and the center electrode may be connected directly to each other by means of the conductive seal material or may be connected such that a resistor is sandwiched between a conductive seal material layer on the side of the metallic terminal member and that on the side of the center electrode. The conductive seal material is generally a mixture of metal and base glass; specifically, metallic particles are dispersed within glass matrix in network-like contact with one another, thereby imparting an electrically conductive property to glass, which in itself is electrically insulative, through assuming the form of a composite material.
In recent years, most insulators for use in spark plugs have been formed of alumina ceramic, which exhibits excellent dielectric strength. Meanwhile, the metallic terminal member or the center electrode is formed of a metal that contains a predominant amount of, for example, Fe or Ni. Thus, the insulator has a coefficient of linear expansion which greatly differs from that of the metallic terminal member or that of the center electrode (e.g., alumina has a coefficient of linear expansion of about 7.3xc3x9710xe2x88x926/xc2x0 C., whereas Fe and Ni have a coefficient of linear expansion of about 12-14xc3x9710xe2x88x926/xc2x0 C.). Therefore, for example, in the course of use, when a spark plug heated to high temperature is cooled, the metallic terminal member or the center electrode contracts by a greater amount than does the insulator. In this case, if the conductive seal material fails to follow the contraction, the material may suffer separation or a like defect. Conventionally, the conductive seal material is a mixture of metal and glass (inorganic material) so as to assume an intermediate coefficient of linear expansion between the coefficient of linear expansion of the insulator and that of the metallic terminal member or the center electrode, thereby reducing a contraction difference therebetween to a certain extent.
However, in recent years, engines to which spark plugs are applied have tended to have high output accompanying an increase in compression ratio of an air-fuel mixture, thereby requiring seal materials to provide higher sealing performance. Further, in recent engines, a mechanism around a cylinder head, on which spark plugs are mounted, has become complicated, and thus a mounting space for spark plugs tends to be narrowed. Therefore, a reduction in spark plug size has been strongly required. A reduction in spark plug size leads to a reduction in insulator size and thus a reduction in the diameter of a through-hole formed in the insulator. Accordingly, when the combustion pressure of an engine is imposed on the center electrode of such a size-reduced spark plug, force per unit area to be imposed on the seal material provided within the through-hole increases. In view of this increase as well as an increase in compression ratio of an air-fuel mixture, conventional specifications of a conductive seal material are becoming insufficient for satisfying durability requirements.
An object of the present invention is to provide a spark plug capable of providing sufficiently high sealing performance by means of a conductive seal material even when the diameter of a through-hole formed in an insulator is small, and capable of achieving sufficient durability even in application to an engine of high output.
In order to achieve the above object, the present invention provides a spark plug in which a metallic terminal member and a center electrode are securely and rigidly held, via a conductive seal material, within a through-hole formed axially in an insulator, characterized in that the insulator is formed of alumina ceramic; the diameter of the through-hole is not greater than 4 mm as measured at a position where the conductive seal material is disposed; and a coefficient of linear expansion of the conductive seal material is adjusted to not greater than 6.8xc3x9710xe2x88x926/xc2x0 C. In the present invention, alumina ceramic contains alumina in an amount not less than 80% by mass, and the coefficient of linear expansion is that obtained by averaging those at 20xc2x0 C.-350xc2x0 C.
As mentioned previously, alumina used to form an insulator has a coefficient of linear expansion of about 7xc3x9710xe2x88x926/xc2x0 C.; and a conventional spark plug employs a conductive seal material (hereinafter may be referred to merely as a seal material) having an intermediate coefficient of linear expansion between the coefficient of linear expansion of alumina and that of a metal used to form a metallic terminal member or a center electrode. In this case, in the course of cooling from high temperature, as shown in FIG. 8(a), the seal material contracts by a greater amount than does the insulator formed of alumina ceramic; as a result, tensile stress, which is induced by differential contraction between the seal material and the insulator, is likely to remain in the seal material at its surface of bonding to the insulator on the inner surface of a through-hole formed in the insulator, resulting in a likelihood of, for example, the seal material being cracked or separated from the insulator. Accordingly, when a small-sized spark plug whose through-hole has a diameter not greater than 4 mm is applied to, for example, an engine to be operated at high output and high compression ratio, the spark plug fails to exhibit sufficient durability. When the seal material contracts radially to a considerable extent, the seal material separates from the inner surface of the through-hole formed in the insulator, possibly resulting in impaired gastightness or impaired durability of the seal material itself.
However, according to a first configuration of the spark plug of the present invention, the coefficient of linear expansion of the seal material is adjusted to be lower than that of alumina; specifically, to be less than 6.8xc3x9710xe2x88x926/xc2x0 C. Therefore, as shown in FIG. 8(b), in the course of cooling, the relationship in amount of contraction between the seal material and the insulator is reversed from that of the conventional spark plug; i.e., compression stress, which is advantageous for suppression of propagation of cracking, remains in the seal material. As a result, even when a small-sized spark plug whose through-hole has a diameter not greater than 4 mm is applied to an engine to be operated at, for example, high output and high compression ratio, a bond portion of the seal material can exhibit sufficient durability, and thus good gastightness can be maintained over a long period of time. Also, radial contraction of the seal material is suppressed, thereby avoiding a likelihood of the seal material being separated from the inner surface of the through-hole formed in the insulator with resultant formation of clearance. Preferably, the coefficient of linear expansion of the seal material is adjusted to not greater than 6.5xc3x9710xe2x88x926/xc2x0 C.
When the coefficient of linear expansion of the seal material is not less than 6.8xc3x9710xe2x88x926/xc2x0 C., the above-described effect is not sufficiently yielded. No particular limitation is imposed on the lower limit of the coefficient of linear expansion of the seal material; however, the lower limit that is attainable through selection of material exists of its own accord. The present inventors have confirmed from studies that there can be implemented a seal material having a coefficient of linear expansion that is lowered to, for example, about 3.0xc3x9710xe2x88x926/xc2x0 C.
The conductive seal material can specifically contain base glass, a conductive filler, and an insulative filler, and, in order to impart the above-described coefficient of liner expansion to the conductive seal material, the insulative filler can contain an inorganic material having a coefficient of linear expansion lower than that of aluminum oxide. Preferably, in order to suppress the coefficient of linear expansion of the conductive seal material to a lower level, the insulative filler is formed of an inorganic material having a coefficient of linear expansion lower than that of the base glass.
As in the case of a conventional conductive seal material, the base glass can be glass that contains a predominant amount of oxide, such as borosilicate glass. In this case, an insulative filler formed of an oxide-type inorganic material exhibits enhanced affinity to the base glass and is thus advantageous for realizing a seal structure of excellent strength and gastightness. For example, one or more substances selected from the group consisting of xcex2-eucryptite, xcex2-spodumene, keatite, silica, mullite, cordierite, zircon, and aluminum titanate can be favorably used as such an oxide-type inorganic material in the present invention.
When an insulative filler to be used is formed of an oxide-type inorganic material having a coefficient of linear expansion lower than that of aluminum oxide, preferably, in the microstructure of the conductive seal material as observed on a cross section thereof, insulative filler particles having a particle size of 100-350 xcexcm occupy an area percentage of 2-40% in the microstructure. Notably, herein, the particle size in the context xe2x80x9cthe particle size of an insulative filler particle as observed in the microstructure of a cross sectionxe2x80x9d is represented by the diameter of a circle having an area identical to that of the particle appearing on the cross section.
Use of an insulative filler formed of an oxide-type inorganic material having a coefficient of linear expansion lower than that of aluminum oxide can appropriately lower the coefficient of linear expansion of the conductive seal material below that of the insulator formed of alumina ceramic, and is thus advantageous for maintaining durability of a bond portion of the seal material. The above-described adjustment of the form of presence of the insulative filler as observed in the microstructure of a cross section of the seal material considerably enhances the sealing performance and durability of the seal material. Thus, for example, even when a small-sized spark plug whose through-hole has a diameter not greater than 4 mm is applied to an engine to be operated at high output and high compression ratio, the spark plug can maintain good gastightness over a long period of time.
When, in the microstructure of the seal material as observed on a cross section thereof, insulative filler particles having a particle size of 100-350 xcexcm occupy an area percentage less than 2%, this indicates that, among insulative filler particles formed of an oxide-type inorganic material, those of small particle sizes (e.g., those having a particle size less than 50 xcexcm) are melted into the base glass in a sealing step performed through application of heat. As a result, the softening point of the seal material increases excessively, with a resultant failure to provide good sealing performance or failure to impart sufficient bonding strength to a bond portion of the seal material. When the area percentage exceeds 40%, this indicates that excessive insulative filler particles are contained, thereby impairing fluidity of the seal material in the course of softening, with a resultant failure to provide good sealing performance or failure to impart sufficient bonding strength to a seal portion.